Do I understand 4x4 wrong?

Depends on who you ask. It does have capability to send power to all four wheels - but some people don’t call it true 4x4 unless there are locking differentials front and rear to guarantee that the power makes it to the ground.

 

Maybe? If the rear was locked then the ground tire would be turning, so if it has some traction, then no it wouldnt be stuck. The mts/crawl control senses one tire turning faster and applies brake to it to transfer torque to its opposing wheel.

 

As TheWildman just said, if you had the rear locked then the torque going to the rear wheel with traction would probably keep you moving. Which is why lockers are unbeatable off-road.

If you had an older vehicle that had no MTS/ATRAC/brake limited slip type of tech, AND also had no rear or front locker, then theoretically yes, you’d be stuck. The front flywheel in the air and the rear wheel in the air would each spin while the two wheels on the ground would just sit there.

 

No. Instead of thinking in terms of power, think in terms of resistance. The junctions that are locked must produce the same speed at each output. If the center is locked, then everything between front and rear differentials are essentially one solid unit. If the center and rear are locked then everything between the front differential and the two rear tires must turn at the same rate, as if they were one unit.

Consider the latter case. 4x4 engaged, rear locked. At least three tires must turn at the same rate, and this will be dependent on whichever front tire is experiencing the least resistance. If you could magically balance the truck on one front tire, the remaining three would spin in the air. However, in order to deliver power to the path of least resistance, again, at least three tires must turn.

4x4 with unlocked rear - at least one front and one rear tire must turn in order to deliver power to the path of least resistance.

2WD - at least one tire must turn in order to deliver power to the path of least resistance.

 

As far as how many tires will always spin, no matter what…
2WD = 1WD
4WD = 2WD.
4WD + rear diff locker = 3WD
4WD + front & rear diff lockers = 4WD.

 

In this situation is the front and back all accounting for only one low resistance wheel? Like if the front tire is low resistance will the back match that? Or is the back independent. Sorry if you kind of answered this just want clarification

 

Hopefully with toyotas computer chip magic all wheels are accounted for. But with front and rear locked diffs there is no doubt that all wheels are turning. Thats if everything is working, Obviously if you broke something in the drivetrain then the broken part wont work as intended.

 

Also theres a thread that has gone on forever, and Im not allowed to comment because I dont have 60 posts.

Would some one please make sure that custards are a votable option in the pie thread?

Please and Thank You!

 

"...compensated by the locking center differential..."

There is no "center differential" to lock on the Tacoma. That is for AWD cars or 'full time' 4WD SUVs that can drive on dry pavement with increased control and traction.
In the center is the transfer case, which switches from 2WD (rear wheel drive) to 4WD. 100% split. Off road or slippery roads only.
When they 'lock' that center diff, they are no longer just a dry weather AWD, and become what we have in a Tacoma, true 4WD.

 

I think you're overthinking it.

Everything in the system can be thought of as one input shaft being split into two output shafts.

Here are the rules of the game. At least one output shaft must turn at the same rate as the input shaft. If you apply resistance to only one output, the less constricted shaft will match the speed of the input. If both outputs are under equal resistance, both will match the speed of the input. Likewise, if both outputs are mechanically locked together, both outputs will turn the same speed as the input.

Start by imagining a 2WD with a locker. The drive shaft is the input, each half of the axle are the outputs. On paved road, both tires are under equal resistance, therefore both are getting power and are turning at the same rate. Now let's go off road. One tire is in a mud hole, the other is on a rock. Nature loves sending power down the path of least resistance. Therefore, the drive shaft (input) turns, and the least resistant tire must also turn at the same rate. This would be the tire in the mud. Now, let's engage the diff lock. Both tires must turn at the same rate as the drive shaft, therefore the tire on the rock moves the truck.

(For the pedants out there, we are pretending diff gear ratios don't exist as they are not pertinent to the point. )

Once you understand that bit, we can expand to a 4x4. Now the transmission will drive the transfer (input), which in turn must drive the rear and/or front driveshafts (outputs). On the Tacoma, these driveshafts are either locked or off. Therefore, when engaged the transfer case follows the rule of "Both locked outputs must turn at the same rate as the input." After this, each shaft now plays the role of input, each differential now the junction to their two output tires. By previously established rule both inputs must turn at the same rate. By the rules, at least one output on each end must also turn at the same rate. This would be one front and one rear tire, assuming all four are under different resistances, these two tires are turning at the same rate as their drive shafts, which are turning at the same rate as the transmission output. If we're on pavement, then again, by the rules, all four tires are receiving equal resistance and therefore are all under equal power.

Once you understand that bit, now we can revisit the locker under 4x4. By the rules, both drive shafts are turning, the rear shaft turning two locked wheels and the front turning at least one wheel at the same speed, again assuming that all four wheels are under different resistances.

I'm hoping this clears the mud a bit.

 

This is where you screwed up. In 4x4 front and rear transfer case outputs are locked together so they rotate at the same speed but the torque is NOT split equally at 50/50. Only an open differential splits torque equally (as you stated) and any two outputs locked can bias torque completely 100/0 in either direction.

 

No, no. Locking diffs do NOT send equal power or torque. They just force equal speeds. It biases torque either direction up to 100/0 and anywhere in between. Power is torque times angular velocity and even though angular velocity is the same the torque isn't necessarily the same so power is therefore not required to be equal.

 

Open diff torque is not always 50/50.

Transfer case is 50/50 because it's a fixed chain.

 

I understand what you're saying, but this is kinda throwing me off. Seems like it would be hard for both to have equal resistance. Wouldn't it typically just be one that has slightly less resistance which is getting the power, and the other is following suit? Or is both getting power common?

 

By that last part, do you mean each axle (front and rear) can give either passenger or driver wheel 100/0 torque? Soboth axle's have 100% to give (since it's locked), not 50% like I originally stated.

 

The ABS system is going to control the "power" to each wheel in low traction terrain.
Unless you're in 4 LO with Traction Control Off, and/or Locked.

 

No. It's technically torque biased anywhere from 100% front, 0% rear to 0% front, 100% rear and whatever torque is delivered to an axle is then equally split to each wheel across the open differential.

One example would be 30% of the torque applied to the front axle and 70% to the rear. In that case it would be 15% to each front and 35% to each rear.

Another good example is assume all wheels have exceptional traction except the LF. Because the LF cannot react torque the total front axle torque becomes very limited and the majority of the driveline torque will be reacted at the rear axle. So the front may get 2% total split to 1% at each front wheel (LF limited) and therefore 98% of the torque is applied to the rear where each rear wheel then gets 49% after equal split through the open diff.

 

No, each *axle* can receive up to 100% of the available torque but it still must split the torque applied to that axle equally to each wheel because of the open diff.

 

Now I'm going to attempt to translate this academic hand waving into the real world. What does all of this really mean?

It's simple. The more wheels you force to link together and spin at the same rate, the higher the odds that at least one will find traction in a sloppy situation. If you engage 4x4 , you know that somewhere at your front and somewhere at your rear are turning rubber, and there's a good chance that at least one tire is finding purchase. If you engage 4x4 with a locker, then at least three corners are turning, and it really takes a bizarre scenario to not have all four turning. It's possible, but it's not common.

This sounds great, one might say. Why aren't they locked together all the time? Well, because locked doesn't guarantee the best solution to every situation. On dry pavement, in the normal course of driving and cornering, all four wheels want to rotate at different speeds. Forcing them to hold hands will cause tire scrub, which will actually introduce the occasional momentary loss in traction. A car taking a sharp corner with a locked rear will easily spin out, where an open diff may hold the line, for example. There is also the issue that scrubbing wheels will cause a lot of wear and tear on the vehicle.

The hardest trick is to look at a terrain and judge which buttons to push. Will you hurt anything by going to 4Lo-locked on a flat gravel road? No. It won't hurt a thing, other than a slight acceleration in the wear. Will you look cool? Also no. But who cares, go out, experiment, make mistakes, and talk to people. That's how we all learn.

 

Intuitively written, technically incorrect.